Footwear is divided into two general parts, an upper and a sole. The upper is designed to snugly and comfortably enclose the foot. The other major portion of a shoe is the sole. The sole must provide traction, protection, and a durable wear surface. The considerable forces generated in athletics require that the sole of footwear provide enhanced protection and shock absorption for the foot and leg. It is also desirable to have enhanced protection and shock absorption for he foot and leg in all types of footwear. Accordingly, the sole of athletic footwear typically includes several layers, including a resilient, shock absorbent material as a midsole and a ground contacting outer sole or outsole, which provides both durability and traction.
Such midsoles have been made of conventional foam materials, for example, ethylene vinyl acetate (EVA) or polyurethane which compress resiliently under an applied load and dampen to provide cushioning. The outsoles have been made of conventional wear-resistant materials such as a carbon-black rubber compound. Conventional foam materials are resiliently compressible, in part, due to the inclusion in the foam of open or closed cells defining an inner volume that is substantially displaced by gas. That is, the foam can include bubbles formed in the material which include air therein. However, after repeated compression, foam materials deteriorate, in particular, by compaction. The cell structure collapses, resulting in decreased compressibility of the foam. Thus, the overall cushioning of the midsole deteriorates.
One way to overcome the drawbacks of using conventional foam materials is disclosed in U.S. Pat. No. 4,183,156 (patented Jan. 15, 1980 to Marion F. Rudy), incorporated by reference, in which cushioning is provided by inflatable inserts made of elastomeric materials. The inserts include a plurality of tubular chambers which extend substantially longitudinally throughout the length of the footwear. The chambers are in fluid communication with each other at the forefoot and jointly extend across the width of the footwear. In U.S. Pat. No. 4,219,945 (patented Sep. 2, 1980 to Marion F. Rudy), incorporated by reference, an inflated insert is encapsulated in a foam material. The combination of the insert and the encapsulating material functions as the midsole. An upper is cemented to the upper surface of the encapsulating material and an outsole or tread member may be fixed to the lower surface.
U.S. Pat. No. 4,817,304 (patented Apr. 4, 1989 to Mark G. Parker, et al.), incorporated by reference, discloses a foam encapsulated gas-filled insert in which gaps are left along the sides of the encapsulating member. When the midsole is compressed, the insert expands into the gaps. Thus, the gaps provide decreased stiffness in compression of the midsole, while reducing the overall weight of the footwear. Further, by appropriately locating the gaps, the overall impact response characteristics can be tuned along areas of the footwear.
Such bladders generally are made of an elastomeric material and are formed so as to have an upper or lower surface enclosing one or more chambers therebetween. The chambers are pressurized above ambient pressure by insertion of a nozzle or needle connected to a fluid pressure source into a fill inlet formed in the bladder. After the chambers are pressurized, the fill inlet is sealed, for example, by welding, and the nozzle is removed.
Bladders of this type have been manufactured by the prior art two-film technique in which two separate sheets of elastomeric film are formed having the overall peripheral shape of the bladder. The sheets are welded together along the periphery to form a bladder having upper, lower and side surfaces, and at predetermined interior areas to give the bladder a preferred configuration, that is, to have chambers of a predetermined shape and size at desired locations.
Bladders have also been manufactured by the prior art blow-molding technique. A liquefied elastomeric material is placed in a mold having the desired overall shape and configuration of the bladder. The mold has an opening at one location through which pressurized air is provided. The pressurized air forces the liquefied elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold to form a bladder having the preferred shape and configuration.
Another type of prior art bladder used in soles of footwear is disclosed in U.S. Pat. No. 4,906,502 (patented Mar. 6, 1990), and U.S. Pat. No. 5,083,361 (patented Jan. 28, 1992), both issued to Marion F. Rudy, and both hereby incorporated by reference. The bladders disclosed in the '502 and '361 patents are formed as a gas pressurized and inflated structure which comprises an hermetically sealed outer covering barrier layer which is securely bonded substantially over the entire outer surfaces of a double walled interior fabric structure. The double walled fabric structure comprises first and second outer fabric layers which are normally spaced apart from one another at a predetermined distance. Connecting or drop yarns, preferably in the form of multi-filament yarns comprised of many individual fibers, extend internally between the proximal or facing surfaces of the respective fabric layers. The filaments of the drop yarns form tensile restraining means and are anchored to the respective fabric layers. The preferred method of manufacturing the double walled fabric structure is double needle bar Raschel knitting.